Method and apparatus for the coding and low temperature liquefaction of gaseous mixtures

ABSTRACT

.Iadd.17. A process for totally liquefying a gaseous methane-rich feed stream comprising the steps of: 
     a. supplying said methane-rich feed stream at a superatmospheric pressure and precooling said stream, 
     b. providing a multicomponent refrigerant, 
     c. compressing said multicomponent refrigerant to a superatmospheric pressure, 
     d. cooling said multicomponent refrigerant and phase separating a single vapor fraction and a single liquid fraction from said cooled multicomponent refrigerant, 
     e. subcooling said liquid fraction in heat exchange with itself after expansion to form a first subcooled liquid fraction, 
     f. liquefying and subcooling all of said vapor fraction in heat exchange with said first subcooled liquid fraction, and with itself after expansion, to form a second subcooled liquid fraction, and 
      g. totally liquefying said precooled methane-rich feed stream by further cooling said precooled methane-rich feed stream to at least its liquefaction temperature, at the superatmospheric pressure thereof, solely by progressive heat exchange steps with said first and second subcooled liquid fractions undergoing vaporization, 
     said methane-rich feed stream and said multicomponent refrigerant being in indirect heat exchange with each other throughout the process..Iaddend. .Iadd.

This application is a division of Reissue application Ser. No. 868,246,filed Sept. 29, 1969, now patented as Re. 29,914, Feb. 20, 1979..Iaddend.

This invention deals with a method and apparatus required for partial ortotal liquefaction with or without sub-cooling of the liquid thusobtained from gas or gas mixtures of which certain components have verylow boiling temperatures at atmospheric pressure. It concerns especiallya method and apparatus for the liquefaction and sub-cooling to a liquidstate of mixtures including light hydrocarbons, nitrogen and helium, forexample, and those which cannot be kept in a liquid state in closedcontainers without refrigeration; that is to say those with a criticaltemperature lower than the ambient temperature. This invention can beespecially applied to natural gas liquefaction, extraction of nitrogen,helium and rare gases.

This invention in its broadest form is not limited to the liquefactionof the gases mentioned hereabove, but applies to any complex gasmixture. It applies especially when it is essential to constructinstallations processing mixtures of variable and varied composition,having therefore to be extremely flexible.

Moreover, this invention permits the construction of installations withlow investment and operating costs.

An important application of this invention includes the liquefaction ofnatural gas produced in locations separated from the place ofconsumption by bodies of water, making transport by pipe-line impossibleor very costly. In this case, the gas is transported by special ships inliquid form at a pressure slightly above atmospheric pressure and at atemperature of about the bubble point; the evaporation of a part of theliquid produces the refrigeration effect required to maintain the cargocold and the gas thus evaporated can be used as fuel for the ship'sengines.

Up to the present the liquefaction of natural gas without expansionsupplying work has been made by, amongst others, the following twomethods:

The first uses several refrigeration cycles, each cycle using anessentially pure refrigerating fluid. For example, such a system can usepropane for the first cycle between the ambient temperature and -35° C.,ethylene for the second cycle between temperatures of -35° C. and -100°C., methane for the third cycle between temperatures of -100° C. and-160° C., and if necessary a fourth cycle with nitrogen to reach atemperature lower than -160° C. In order to lessen the energyconsumption, each cycle of this system consists itself of severaltemperature stages, these being obtained by controlling at differentlevels, the boiling pressure of the refrigerating fluid. The consequenceof this is an expensive multiplication of equipment: heat exchangers,piping, control equipment, compressors. This complexity leads to highinvestment costs. Moreover this system is not very flexible and isdifficult to adjust as soon as the real conditions vary a little fromthe basic conditions.

The second method uses the gas itself as the refrigeration fluid;however, this other solution requires important auxiliary equipmentwhich considerably increases the complexity of the system and the powerrequired to accomplish liquefaction.

The principal object of the present invention is to provide a method andapparatus for carrying out continuous, total, or partial liquefaction,with or without sub-cooling, of the liquid thus obtained from thegaseous mixtures, whose critical temperature is lower than the ambienttemperature.

Another object of the present invention is to provide a method andapparatus for carrying out continuous liquefaction of gaseous mixtures,whose composition can vary from time to time, and this under conditionsnear to the optimum for a fairly wide range of variations.

Another object of the present invention is to provide apparatus forcarrying out the continuous liquefaction of gaseous mixtures withminimum capital and operation costs.

A specific object of the present invention is to provide a method forthe continuous liquefaction of gaseous mixtures in which the coldproducing fluid or refrigerating fluid is a mixture of severalcomponents which mixture is chosen to allow the liquefaction of thegaseous mixtures under the most efficient conditions.

Another object of the present invention is to provide an apparatus forcarrying out the liquefaction of gaseous mixtures in which thecomposition of the refrigerating fluid can be modified so as to maintainthe operating conditions close to the optimum.

Another object of the present invention is to provide a method andapparatus for continuous, total, or partial liquefaction, with orwithout sub-cooling of the liquid thus obtained from the gaseousmixtures by counter-current surface heat exchange with a refrigeratingfluid of varying composition at several points in the cooling circuit,the refrigerating fluid itself being, when compressed, a gaseous mixturedistinct from the mixture to be liquefied, this fluid being continuouslyrecirculated, while the gas to be liquefied only passes through theexchanger system once.

Another object of the present invention is to provide a method and anapparatus permitting the continuous cooling, the condensation under acertain pressure of the natural gas and its sub-cooling to a liquidstate down to a temperature near or equal to its bubble temperature atatmospheric pressure by counter-current indirect heat exchange betweensaid natural gas and said fractions of a refrigerating fluid comprisingmainly methane, ethane, propane, butanes, pentanes, but also possiblycontaining small quantities of more volatile substances, nitrogen andhelium, for example, and less volatile substances, such as hexanes,heptanes, octanes. This is an inexpensive cold producing fluid(refrigerating fluid) easily obtained since it is made up of thehydrocarbons of the gases contained in the natural gas or in the liquidusually present in the natural gas before its processing in the field,prior to its transport by pipe-line.

The different objects are obtained in accordance with the invention byway of a process of liquefaction of a mixture of gas hereafter called a"first gas mixture," characterized by the fact that this gas mixturepasses through a first system of liquefaction, in indirect contact withand in a direction opposite to a refrigerating liquid in the process ofvaporization, introduced in portions at several points, these portionsbeing increasingly volatile in the direction followed by the first gasmixture, said portions of refrigerating liquid resulting from theliquefaction in successive stages, into a second liquefaction system, ofa second gas mixture obtained by vaporization of the refrigeratingliquid in said systems of liquefaction, this second liquefactionresulting from the vaporization with indirect contact of at least onepart of the condensate of each previous stage, with the exception of thefirst condensate which is obtained by compression of the second gasmixture, the pressure of vaporization in the first liquefaction systembeing lower than the pressure of vaporization in the second liquefactionsystem.

The absolute vaporization pressure in the first liquefaction system willusually be between about 0.4 and about 3 atmospheres, preferably betweenabout 1 and about 1.5 atmospheres.

The absolute vaporization pressure in the second liquefaction systemwill usually be between about 4 and about 8 atmospheres, preferablybetween about 5 and about 7 atmospheres.

The absolute liquefaction pressure of the first gas mixture ispreferably between about 30 and about 100 bars, whereas the absoluteliquefaction pressure of the second gas mixture is usually selected tobe higher than about 31 bars, for example between about 31 and about 50bars.

The adoption of a vaporization pressure in the second liquefactionsystem which is higher than that of the first system, constitutes anessential feature of the invention.

In fact, under these conditions, power consumption is greatly reduced.

On the other hand, the losses in heat are reduced on account of the factthat the liquefaction in the second liquefaction system is effected atrelatively high temperatures.

Finally, this difference in pressure makes it possible to utilize a coldproducing fluid of a low nitrogen content or a low content of anothergas having a low boiling point. The liquefaction of such a fluidrequires a less high power consumption.

Another preferred feature of the invention is the fact that the gasmixture is circulated from below moving upwardly, and the refrigeratingfluid downwardly from above, throughout the first and-or secondliquefaction system. Such manner of procedure provides a number ofpractical advantages. In particular, pipes of large volume in which thegas circulates, can be placed near the ground, close to the compressor.These pipes may be short which reduces the cost of installation andoperation by reducing the losses in load. Only pipes of a small diameterdestined for liquified gases are connected to the highest parts of thesystem. This arrangement is also more advantageous than the use ofexchangers arranged successively at the same level. In fact, in thelatter case, connecting pipes between the different apparatuses must beprovided which are fairly long, for example for letting the partiallyliquified gas rise again from one level to the next.

Another preferred feature resides in refrigerating fluids of particularcomposition.

The composition of the cold producing fluid may vary somewhat with thecomposition of the gas to be liquefied and with the liquefactionpressure; in the same manner, the nitrogen content of the cold producingfluid depends upon the temperature at which the liquefied natural gas isto be sub-cooled.

Generally, the refrigerating fluids have the following composition inthe gaseous state:

    ______________________________________                                                                Percent                                                                       by volume                                             ______________________________________                                        Nitrogen and more highly volatile gases (for exam-                             ple helium)              0-3                                                 Methane                   20-32                                               Ethane                    34-44                                               Propane                   12-20                                               Butanes                    8-15                                               Pentanes and heavier fluids                                                                             3-8                                                 ______________________________________                                    

According to a preferred form of application, the content of nitrogenand more volatiles is between about 0.5 and about 2%.

Refrigerating fluids of the composition given above, can be utilized inpractically all cases of liquefaction of natural cases containingmethane as the main component. With such refrigerating fluids, powerconsumption is reduced to a minimum.

These and other objects of this invention will become apparent in thwefollowing description of the invention as it could be applied, forexample, to the liquefaction of a natural gas in view of itstransportation by sea or river ships.

For example, the gas could have the following composition and beavailable at a pressure of 39 bars absolute:

    ______________________________________                                                           Percent by volume                                          ______________________________________                                        Nitrogen             1.8                                                      Methane              87.0                                                     Ethane               7.3                                                      Propane              2.5                                                      Butanes              1.2                                                      Pentanes and less volatiles                                                                        0.2                                                      ______________________________________                                    

FIG. 1 illustrates a simple application of the invention.

FIG. 2 shows the heat-temperature relationship between the processednatural gas and the cold-producing fluid or refrigerating fluid.

FIG. 3 shows, in order to allow the comparison, the same relation asFIG. 2, in the case of a three cycle cold-producing process.

FIG. 4 illustrates a practical example of an application of theinvention.

FIG. 1 illustrates a simple application of the invention. Arefrigerating fluid, after being compressed in compressor 1 up to anabsolute pressure of 37.5 bars, for example, is admitted through line 2into the indirect contact surface condenser 3 (wherein air or water isused for example). The fluid is partially condensed when it comes out ofcondenser 3 through line 4. It is then passed to drum 5. Liquidcondensate called L1 is collected in drum 5 while the vapor in drum 5 isintroduced through line 6 into the tubes of condenser 7 from which itcomes out partially condensed through line 8, for passage to drum 9.Liquid condensate called L2 is collected in drum 9 while the vapor ofthe fluid in line 8 is introduced through line 10 into the tubes ofcondenser 11, from which it comes out partially condensed through line12 and through which line it passed to drum 13. Liquid condensate calledL3 is collected in drum 13 while the vapor therein is introduced throughline 14 into the tubes of condenser 15 from which it comes outcompletely condensed through line 16; this condensate, called L4 feedsthe shell of exchanger 36.

The stepwise condensation of the cold producing fluid is obtained in thefollowing manner. A part of the condensate L3 through lines 17 and 18 isinjected through expansion valve 19 into the top of the shell ofcondenser 15 wherein it partially vaporizes while descendingcounter-currently with the fluid condensing within the tubes ofcondenser 15. From the bottom of the shell of condenser 15, partiallyvaporized L3 is introduced into the top of the shell of condenser 11through line 20. A part of the condensate L2 from lines 21 and 22 isinjected through expansion valve 23 into line 20. The resulting mixtureof L3 and L2 is passed into the shell of condenser 11 wherein itvaporizes while descending counter-currently with the fluid condensingwithin the tubes of 11. From the bottom of the shell of condenser 11,the vaporized L2-L3 mixture is passed through line 24 and is introducedinto the top of the shell of condenser 7. A part of the condensate L1,in lines 25 and 26, is injected through expansion valve 27, and is mixedwith the vapor in line 24. The resulting mixture of L3, L2 and L1vaporizes and is superheated while descending counter-currently with thefluid condensing in the tubes of condenser 7. From the bottom of theshell of condenser 7, the mixture of L3-L2-L1 in a superheated vaporstate is introduced through line 28 to an intake of compressor 1.

The natural gas to be liquefied enters through line 29 into the tubes ofexchanger 30, then successively through line 31 into the tubes ofexchanger 32, then through line 33 into the tubes of exchanger 34, thenthrough line 35 into the tubes of exchanger 36, from which it comes outin a sub-cooled liquid state through line 37, then through valve 38 andline 39 to storage. All the exchangers are wall-contact type. Thecondensate L4 passing through line 16 and expansion valve 40 is injectedinto the top of the shell of exchanger 36, in which it partiallyvaporizes counter-currently with the natural gas circulating in thetubes thereof. From the bottom of the shell of exchanger 36, condensateL4 is introduced into the top of the shell of exchanger 34 through line41. In line 41, a part of the condensate L3 in lines 17 and 42 isinjected through expansion valve 43. The mixture of L4 and L3 vaporizeswhile descending counter-currently with the natural gas circulatinginside of the tubes of exchanger 34. From the bottom of exchanger 34,the mixture of L4 and L3 is introduced into the top of the shell ofexchanger 32 through line 44. A portion of the condensate L2, from lines21 and 45, is injected through expansion valve 46 into line 44. Themixture of L4-L3-L2 vaporizes while descending counter-currently withthe natural gas circulating inside of the tubes of exchanger 32. Fromthe bottom of the shell of exchanger 32, the mixture L4-L3-L2 isintroduced into the top of the shell of exchanger 30 through line 47.Part of the condensate L1 in lines 25 and 48 is injected throughexpansion valve 49 into line 47. The mixture of L4-L3-L2-L1 vaporizesand is superheated while descending counter-currently with the naturalgas circulating in the tubes of exchanger 30. The mixture ofL4-L3-L2-L1, in a superheated vapor state, is passed from the bottom ofexchanger 30 through line 50 to an intake of the compressor 1.

It should be understood that, instead of the 4 exchangers 30, 32, 34 and36, a smaller number of exchangers can be used. For example, a singleexchanger of much greater length with the same number of injections ofthe cold-producing fluids (refrigerating fluids) in lines 16, 42, 45 and48 and the expansion valves 40, 43, 46 and 49 can be used. In this casethe connecting lines 41, 44 and 47 are unnecessary.

It should be understood that, instead of the three condensers 7, 11 and15, only one condenser, for example, can be used in which the condenserbundles would be stacked, and the condenser shell would have the samenumber of points for the injection of the cooling fluids through lines26, 22 and 18, and expansion valves 27, 23 and 19. The connecting lines24 and 20 would then be unnecessary. These exchangers and condensers canbe, for example, coiled tube type or multiple plate type exchangers.

The two refrigeration circuits for cooling and condensation of thenatural gas on the one hand and for condensation of the condensates ofthe cold-producing fluid on the other hand, operate under differentpressures, the first one at a lower pressure than the second one. Sincethe pressure is not the same in the two circuits, two or threecompression casings can be used on the same drive shaft withintermediary cooling if necessary, the regulation of the lowest suctionpressure being made, for example, by means of variable inlet guidevanes. This system has the advantage of only requiring one drivingmachine.

Curve C in FIG. 2 represents the evolution of natural gas under acertain pressure. The quantity of heat called enthalpy (H) is shown asthe abscissa and temperature (T) while cooling is shown as the ordinate.This curve is characterized by three special sections: the first onebetween the ambient temperature (A) and the dew point (B) corresponds tothe gas cooling while in the vapor state; the second section between thedew point (B) and the bubble point (D) represents the progressivecondensation of the vapor, the mixture being entirely liquid at thebubble point temperature; and the third section below the bubble pointtemperature represents the sub-cooling of the liquid obtained bycondensation.

Curve F in FIG. 2 represents under the pressure of vaporization of thecondensates of the cold-producing fluid, the variation of enthalpy of acold-producing fluid taken, as an example, in terms of the vaporizationtemperature. This curve shows a certain number of angular pointscorresponding to the injection points of the condensates points 1 and 2,and point 3 corresponds to the beginning of the superheating of thecold-producing fluid.

The work of cooling of the natural gas is linked to the cross-hatchedarea in FIG. 2; work increases when the surface of this area increases.The minimum work is obtained when curve F merges with curve C but thiswould correspond to an infinite heat exchange surface because thedifference of temperature would be zero at every point. On the otherhand a curve F, whose temperatures at each point are markedly lower thanthe corresponding temperature of curve C, leads to a limited exchangesurface but to increased work.

In order to facilitate the comparison between this process and the wellknown three cycle cold-producing process, FIG. 3 represents, using thesame curve C as in FIG. 2, the operation of a well known process usingthree cooling cycles, each cycle itself being divided into threepressure stages, the enthalpy temperature curve of the cold-producingfluids is a step-like curve requiring in the case shown here ninedifferent exchangers. For example, cycle J relates to prepare, cycle Kto ethylene and cycle L to methane. The three pressure stages of eachcycle are numbered 1, 2 and 3.

The pressure to which the gas is available at the entry of theliquefaction installation has an influence on the power required forliquefying the gas and sub-cooling it.

If it is wished to sub-cool a natural gas in the liquid state down to atemperature near or equal to its bubble point temperature at atmosphericpressure, a pressure more or less equal to the pressure at which, ingeneral, liquefied natural gas is stored, it is necessary, either todrop the pressure of the cold-producing circuit to allow the condensaterich in methane to do this cooling, which adds to the power required forcompression, or to incorporate in the cold-producing fluid a certainquantity of a more volatile component than methane in order to lower thevaporization temperatures in the temperature zone under consideration.This more volatile component can be nitrogen, for example, whose boilingtemperature at atmospheric pressure is -195.8° Celsius.

Due to the concentration action of the successive condensation steps arelatively small quantity of nitrogen, up to 2%, in the cold-producingfluid at the compressor discharge will produce much largerconcentrations in the last and final condensate, so that it is possibleat the same pressure to considerably lower the bubble point temperature,15° Celsius for example. The presence of nitrogen modifies thevaporization curve in its coldest position by removing the almostconstant step existing in the vaporization curves of fractions rich inmethane without more volatile components, but to take full advantage ofthis new form of curve, the liquid condensate must be sub-cooled down toa very low temperature, practically down to the outlet temperature ofthe liquefied natural gas.

The discharge pressure of the compressor also determines the compositionof the cold-producing fluid especially with regards to the quantities ofhydrocarbons heavier than propane. Indeed, the condensation temperatureof the first condensate being determined by the nature and temperatureof the natural fluid for final heat removal (air or water, for example),the composition of the cold-producing fluid must be such that thepressure prevailing in the cold-producing circuit at condenser 3 in FIG.1 (pressure directly linked to that of the compressor discharge), thequantity of condensate L1 collected in the drum 5 should be able tomeet, on the one hand, the refrigeration requirements of condenser 7, ina mixture with the fractions of L2 and L3 injected in the coldercondensers of this circuit, and on the other hand, if necessary, therefrigeration requirements of the exchanger 30 in a mixture with thefractions of L2, L3 and L4 injected in the colder exchangers of thiscircuit counter-currently with the natural gas.

This is obtained by introducing hydrocarbons heavier than propane intothe cold-producing fluid. Introduction of a certain amount of butanesappears, at first sight, to be sufficient to do this, however, a closerlook at the problem, discloses that it would be more advantageous toreplace a part of these butanes by pentanes, and even heavier, hexanesand heptanes, which when present in smaller quantities produce the sameeffect for this precise point and have the advantage of improving theliquid vapor separation of the other components through a betterselectivity of said separation.

The discharge pressure of the compressor, pressure which determines thecondensation pressures of the successive liquid condensates, is linkedto the suction pressure of the cold-producing fluid ensuring thesecondensations-suctions which in effect determines the vaporizationpressures of the condensate mixtures through several actions. The firstone through the compression ratio which determines the power requiredfor the condensations of the liquid condensates of the cold-producingfluid. The others are concerned with the heat exchange in the condensersbetween the condensing vapors and the vaporizing or superheating of thecold-producing fluids. The complete condensation of the residuary vaporand a part of the sub-cooling of this condensate in condenser 15 is doneby vaporization of condensate L3 coming from the same drum 13 as saidvapor. This partly or completely vaporized condensate leaves condensor15 at a temperature necessarily lower than the temperature of the vaporand liquid in drum 13.

Under a vaporization pressure sufficiently low, the condensate leavescondenser 15 completely in a state of super heated vapor. At a pressurehigher than the preceding one (condenser 11), the condensate comes out,at its dew point temperature. At any pressure above the latter thecondensate comes out partially vaporized, the liquid fraction vaporizingonly when mixed with L2 in condenser 11.

Therefore, as the vaporization pressure increases, thus reducing thecompression ratio and therefore the power for an equal out-put, thequantity of the condensate L3 slightly increases, up to thecorresponding pressure at the outlet to its dew point; then increasesrelatively more quickly when the vaporized percentage at the outletdecreases. The reduction of the compression ratio is counter-balanced bythe increase in the out-put of L3, which also leads to an increase inthe out-put of L2, then an increase of L1.

This system its started-up by introducing in drum 5 propane and theheavier hydrocarbons in liquid form. These are easily obtainedcommercially and it is no problem to keep them liquid under pressure atthe ambient temperature, afterwards it is only necessary to add theother components in gaseous form. These are easily extracted from thenatural gas to be liquefied, in order to progressively put intooperation the system for cold-producing fluid condensation and then thenatural gas liquefaction system.

For the best operation of the system the composition of thecold-producing fluid must be varied according to the seasonal variationsof the heat evacuation agent (water or air); for example, the heavyhydrocarbons content must be increased in summer as against thatnecessary during winter.

Contrary to processes using several refrigeration cycles withessentially pure components and which lack flexibility, the process ofthe present invention can be easily adapted to any changes in thenatural gas composition or pressure, by slight modification of thecold-producing fluid composition.

A practical example is given below of an application of the inventionillustrated in FIG. 4.

The natural gas could be defined as follows and be available at apressure of 39 bars, for example:

    ______________________________________                                                           Percent by volume                                          ______________________________________                                        Nitrogen             1.8                                                      Methane              87.0                                                     Ethane               7.3                                                      Propane              2.5                                                      Butanes              1.2                                                      Pentanes and less volatile fluids                                                                  0.2                                                      ______________________________________                                    

It is desired to obtain the liquid sub-cooled at the temperature of-154° C.; the cold-producing fluid composition would be as follows:

    ______________________________________                                                           Percent by volume                                          ______________________________________                                        Nitrogen             0.5                                                      Methane              26.5                                                     Ethane               39.6                                                     Propane              16.1                                                     Butanes              12.6                                                     Pentanes and less volatile fluids                                                                  4.7                                                      ______________________________________                                    

The cold-producing fluid having the composition indicated above leavescompressor 101 under an absolute pressure of 37.5 bars and at atemperature of about 100° C. through line 102 towards thewater-condenser 103 which it leaves partially condensed through line104. Liquid condensate L1 is recovered in drum 105 at the absolutepressure of 37 bars and a temperature of 40° C. Vapor in drum 105 isintroduced through line 106 into the partial condenser 107, from whichit comes out partially condensed through line 108. Liquid condensate L2is recovered in drum 109 at an absolute pressure of 36.5 bars and atemperature of -6° C. Vapor in drum 109 is introduced into condenser111, through line 110, from which it comes out, partially condensed,through line 112. Liquid condensate L3 is recovered in drum 113 at anabsolute pressure of 36 bars and a temperature of -66° C. Vapor in drum113 is introduced through line 114 into condenser 115, wherein thisvapor is entirely condensed and the liquid so obtained is sub-cooleddown to the temperature of -123° C. under an absolute pressure of 35.5bars. Condensate L4 leaves the condenser 115 through line 116. It is tobe noted that the condensation takes place at an approximately constantpressure.

The condensations are achieved in the following manner. Condensate L3recovered in drum 113 is introduced into the tubular bundle located inthe shell of the condenser 115, from which it comes out cooled at thetemperature of -123° C., through line 118. One part of this sub-cooledcondensate is injected through line 119 and expansion valve 120 into theupper part of the condenser shell of condenser 115 wherein there is anabsolute pressure of 4 bars. The sub-cooled condensate is vaporizedpartially while gravitating counter-currently with the two fluids in thetubular bundles in condenser 115 and comes out of the lower end of thisshell nearly totally vaporized at a temperature of -72° C.; it is thenintroduced into the upper part of the condenser shell 111 through line121.

One portion of condensate L2 is recovered in the drum 109 and isintroduced through line 122 into a tubular bundle located in condenser111 from which it comes out sub-cooled at the temperature of -66° C.through line 123 to be injected through expansion valve 124 into line121 wherein it mixes with the fraction L3. The temperature of theliquid-vapor mixture so obtained is -75° C. The liquid-vapor mixture isintroduced into the upper part of the condenser shell of condenser 111and is vaporized counter-currently with both fluids in the tubularbundles in condenser 111. The vaporized mixture comes out of the lowerend of this shell in the vapor state at the temperature of -13° C. andproceeds through line 125 towards the upper part of the shell ofcondenser 107.

Condensate L1 recovered in drum 105 is passed through line 126 and theexpansion valve 127, and is injected into line 125 in which it is mixedwith the vapor coming out of the shell of the condenser 111. Thetemperature of the liquid-vapor mixture thus obtained is -18° C. Thismixture is vaporized and is superheated while gravitating in the shellof condenser 107 counter-currently with the condensing fluid and comesout at the bottom of the shell of condenser 107 at a temperature of 27°C. through line 128 towards the intake of compressor 101.

The natural gas is introduced through line 129 under an absolutepressure of 39 bars and a temperature of 38° C. into the tubular bundleof exchanger 130 from which it comes out at the temperature of -6° C. ina vapor state. The vapor then passes through line 131 into tubularbundle of exchanger 132, from which it comes out in liuqid state at atemperature of -123° C. The liquid then passes through line 133 into thetubular bundle of exchanger 134, from which it comes out at thetemperature of -143° C., thence through line 135 into the tubular bundleof exchanger 136, from which it comes out through line 137 at thetemperature of -154° C. and thence through expansion valve 138 and line139 to storage.

The cooling of natural gas is carried out as follows. The most volatilecondensate leaves condenser 115 at a temperature of -123° C., throughline 116; it is sub-cooled in a tubular bundle of exchanger 134, fromwhich it comes out at a temperature of -143° C. through line 140. It issub-cooled again in a tubular bundle of exchanger 136, from which itcomes out at the temperature of -154° C. through line 141. It isafterwards expanded through valve 142 in the upper part of the shell ofexchanger 136, where it enters at the temperature of -159° C. under anabsolute pressure of 2 bars. It is partially vaporized while gravitatingwithin the shell counter-currently with the two fluids in the process ofbeing cooled in exchanger 136 to come out at the bottom of the shell of136 partially vaporized at a temperature of -150° C. It is thenintroduced into the upper part of the shell of exchanger 134 throughline 143.

Condensate L3 has been sub-cooled down to -123° C. in exchanger 115. Thefraction of this condensate which is not injected in the exchanger 115through line 119 and valve 120 is introduced via line 118 into thetubular bundle of exchanger 134, from which it comes out at atemperature of -143° C. through line 145, in order to be injectedthrough expansion valve 146 into line 143. The liquid-vapor mixture soobtained has a temperature of -147° C. It is vaporized partially whilegravitating into the shell of exchanger 134 counter-currently with threefluids being cooled therein, and it comes out at the bottom of the shellof 134, partially vaporized, at a temperature of -131° C. It is thenpassed to the upper part of the shell of exchanger 132 through line 147.

Condensate L2 collected in drum 109 has been partially used up incondenser 111. The remainder of it is introduced via line 148 into thetubular bundle of exchanger 132 from which it comes out sub-cooled atthe temperature of -123° C. through line 149 to be injected throughexpansion valve 150 into line 147. The liquid-vapor mixture so obtainedhas a temperature of -128° C. It is vaporized and superheated whilegravitating counter-currently with two fluids being cooled in exchanger132 and comes out of the bottom of the shell of 132 in the vapor stateat the temperature of -134° C. It is then introduced via line 151 intothe top of the shell of the exchanger 130 wherein it is superheatedcounter-currently with natural gas and comes out strongly superheated atthe bottom of the shell through line 152 towards the suction ofcompressor 101.

At the suction end of the compressor 101 in line 152, the absolutepressure is about 1.5 bars corresponding to the pressure at the top ofthe shell of exchanger 136, that is 2 bars minus the pressure dropthrough the cold-producing circuit. Consequently, one can see that thecold-producing fluid circulates at an almost constant pressure. At thesuction end of compressor 101 in line 128, the absolute pressure isapproximately 3.5 bars corresponding to the pressure at the top of theshell of condenser 115, that is 4 bars minus the pressure drop in thiscold-producing circuit. Consequently, the cold-producing fluid hascirculated at an almost constant pressure.

It should be understood that, instead of 4 exchangers 130, 132, 134,136, it is possible to use a lesser number of shells by grouping thebundles of several exchangers within the same shell. For instance, onesingle shell could be used comprising the same number of injections ofcold-producing condensates through lines 141, 145 and 149 and expansionvalves 142, 146 and 150. Connecting lines 143, 147 and 151 are thenunnecessary.

It should be understood also that, instead of 3 condensers 107, 111 and115, one single shell could be used, for example, one single shell inwhich the tubular bundles would be stacked. In such a shell, there wouldbe achieved the same number of injections of cold-producing condensatesthrough lines 119, 123 and 126 and expansion valves 120, 124 and 127.Connecting lines 121 and 125 are then unnecessary.

It is advantageous to position in line 135, between exchangers 134 and136, a flash drum (not shown) wherein gases of low boiling point, suchas helium, and other gaseous impurities may be removed from the materialin line 135 through an overhead line (not shown) of this flash drum. Thecondensate formed in this flash drum is passed upwardly through line135.

It should be understood that the diagram on FIG. 4 is a specific exampleand that the sub-cooling of liquid condensates in particular can becarried out in several different ways.

It is to be understood that the cold-producing fluid (i.e.,refrigerating fluid) used herein, distinguishes from natural gas asfound in nature. Rather, components of natural gas can be used toprepare novel mixtures of different composition than natural gas andsuch compositions are suitable for the process of this invention. It isadvantageous, therefore, that all or substantially all of the componentsof the cold-producing fluids can be obtained from natural gas in orderthat the fluids can be prepared; and when nitrogen is a component of thefluids, nitrogen can readily be obtained from air.

I claim: .[.1. Process for the liquefaction of a first gas mixturecomprising passing said first gas mixture through a first liquefactionsystem in indirect contact with and in a direction opposite to arefrigerating liquid in a process of vaporization, said refrigeratingliquid being admitted as a plurality of portions at several points insaid first liquefaction system, said portions being increasingly morevolatile in the direction followed by said first gas mixture and saidportions of refrigerating liquid resulting from a liquefaction insuccessive stages in a second liquefaction system, of a second gasmixture obtained by vaporization of the refrigerating liquid in saidliquefaction systems, said liquefaction in said second liquefactionsystem resulting from the vaporization with indirect contact of at leastone part of the condensate of each previous stage, with the exception ofthe first condensate which is obtained by compression of said second gasmixture, the vaporization pressure of the refrigerating liquid in saidfirst liquefaction system being lower than the vaporization pressure insaid second liquefaction system, said first gaseous mixture flowing thruthe first liquefaction system being out of contact with the fluid in thesecond liquefaction system throughout the process..]. .[.2. Processaccording to claim 1, wherein the absolute vaporization pressure in saidfirst liquefaction system is between about 0.4 and about 3atmospheres..]. .[.3. Process according to claim 1, wherein the absolutevaporization pressure in said first liquefaction system is between about1 and about 1.5 atmospheres..]. .[.4. Process according to claim 1,wherein the absolute vaporization pressure in said second liquefactionsystem is between about 4 and about 8 atmospheres..]. .[.5. Processaccording to claim 1, wherein the absolute vaporization pressure in saidsecond liquefaction system is between about 5 and about 7atmospheres..]. .[.6. Process according to claim 1, wherein the absolutevaporization pressure is approximately 1.5-2 atmospheres in said firstliquefaction system and approximately 3.5-4 atmospheres in said secondliquefaction system..]. .[.7. Process according to claim 1, wherein theabsolute liquefaction pressure of said second gas mixture is greaterthan about 31 bars..]. .[.8. Process according to claim 1, wherein theabsolute liquefaction pressure of said second gas mixture is betweenabout 31 and about 50 bars..]. .[.9. Process according to claim 1,wherein the absolute liquefaction pressure of said fitst gas mixture isbetween about 30 and about 100 bars..]. .[.10. Process according toclaim 1, wherein said first gas mixture circulates upwardly from belowand said refrigerating liquid downwardly from above, with indirectcontact with said first gas mixture through the entire firstliquefaction system..]. .[.11. Process according to claim 1, whereinsaid first gas mixture is a natural gas containing methane; and saidsecond gas mixture contains approximately 0-3% by volume of gas having avolatility which is at least equal to that of nitrogen, 20-32% by volumeof methane, 34-44% by volume of ethane, 12-20% by volume of propane,8-15% by volume of butanes, and 3-8% by volume of hydrocarbons heavierthan the butanes..]. .[.12. Process according to claim 1 wherein atleast one of the liquefaction products of said second liquefactionsystem is sub-cooled before being utilized as a refrigerating liquid inone of said liquefaction systems..]. .[.13. Process according to claim1, wherein the first at least partially liquified gas mixture issubjected to expansion permitting to recover a gas having a lowliquefaction point, before being subjected to a final cooling..]. .[.14.Apparatus adapted for liquefaction of gaseous mixtures comprising: .[. .Apparatus according to claim 14 wherein the heat exchange systemscomprise internal piping adapted for the circulation of saidrefrigerating liquids by gravity..]. .[.16. Apparatus according to claim14, wherein an expansion and separation means for gas and liquid isinterposed in said first exchange system on the path followed by thefirst gas mixture..]. .Iadd.
 17. A process for totally liquefying agaseous methane-rich feed stream comprising the steps of:a. supplyingsaid methane-rich feed stream at a superatmospheric pressure andprecooling said stream, b. providing a multicomponent refrigerant, c.compressing said multicomponent refrigerant to a superatmosphericpressure, d. cooling said multicomponent refrigerant and phaseseparating a single vapor fraction and a single liquid fraction fromsaid cooled multicomponent refrigerant, e. subcooling said liquidfraction in heat exchange with itself after expansion to form a firstsubcooled liquid fraction, f. liquefying and subcooling all of saidvapor fraction in heat exchange with said first subcooled liquidfraction, and with itself after expansion, to form a second subcooledliquid fraction, and g. totally liquefying said precooled methane-richfeed stream by further cooling said precooled methane-rich feed streamto at least its liquefaction temperature, at the superatmosphericpressure thereof, solely by progressive heat exchange steps with saidfirst and second subcooled liquid fractions undergoing vaporization,saidmethane-rich feed stream and said multicomponent refrigerant being inindirect heat exchange with each other throughout the process. .Iaddend..Iadd.
 18. The process as claimed in claim 17, wherein both of saidvaporized liquid fractions are recycled for recompression to saidsuperatmospheric pressure. .Iaddend..Iadd.
 19. The process as claimed inclaim 18, wherein said totally liquefied methane-rich feed stream isexpanded. .Iadd.
 20. The process as claimed in claim 17, furtherincluding the step of maintaining a multicomponent refrigerantcomposition comprising

    ______________________________________                                                            % by volume                                               ______________________________________                                        N.sub.2 and more highly volatile gases                                                              0-3                                                     methane               20-32                                                   ethane                34-44                                                   propane               12-20                                                   butanes                8-15                                                   pentanes and heavier fluids                                                                         3-8                                                     ______________________________________                                          .Iadd.
 21. The process as claimed in claim 17, wherein said     multicomponent refrigerant is compressed to a superatmospheric pressure     between about 31 and about 50 bars. .Iaddend..Iadd.
 22. A process for     liquefying the major portion of a gaseous, methane-rich feed stream     comprising the steps of

a. supplying said methane-rich feed stream at a superatmosphericpressure and precooling said stream, b. providing a multicomponentrefrigerant at a superatmospheric pressure, c. cooling saidmulticomponent refrigerant and phase separating a single vapor fractionand a single liquid fraction from said cooled multicomponentrefrigerant, d. subcooling said liquid fraction in heat exchange withitself after expansion to form a first subcooled liquid fraction, e.liquefying and subcooling all of said vapor fraction in heat exchangewith said first subcooled liquid fraction, and with itself afterexpansion, to form a second subcooled liquid fraction, and f. liquefyingat least the major portion of said methane-rich feed stream by furthercooling said precooled methane-rich feed stream to a temperature belowabout -143° C. solely by progressive heat exchange with said first andsecond subcooled liquid fractions undergoing vaporization,saidmethane-rich feed stream and said multicomponent refrigerant being inindirect heat exchange with each other throughout the process. .Iaddend..Iadd.
 23. The process as claimed in claim 22, wherein both of saidvaporized liquid fractions are recycled for recompression to saidsuperatmospheric pressure. .Iaddend..Iadd.
 24. The process as claimed inclaim 23, wherein said liquefied methane-rich feed stream is expanded..Iaddend. .Iadd.
 25. The process as claimed in claim 22, wherein saidmulticomponent refrigerant is compressed to a superatmospheric pressurebetween about 31 and about 50 bars. .Iaddend. .Iadd.
 26. A refrigerationsystem for totally liquefying a gaseous methane-rich feed stream atsuperatmospheric pressure comprising in combination a. a firstmulti-stage heat exchanger means connected to said free stream forprecooling said feed stream, b. means for supplying a multicomponentrefrigerant, c. a compressor for compressing said multicomponentrefrigerant to a superatmospheric pressure, d. a second multi-stage heatexchanger means for cooling and for partially condensing saidmulticomponent refrigerant, e. a phase separator connected to saidsecond multi-stage heat exchanger means for separating said partiallycondensed multicomponent refrigerant into a vapor fraction and acondensed liquid fraction, f. third heat exchange means connected tosaid phase separator and including expansion means for subcooling saidcondensed liquid fraction in heat exchange with itself, after expansionin said expansion means, to form a first subcooled liquid fraction, g.fourth heat exchanger means connected to said phase separator means andincluding expansion means for liquefying and subcooling said vaporfraction in heat exchange with said first subcooled liquid fraction, andwith itself after expansion in said expansion means, to form a secondsubcooled liquid fraction, and h. fifth heat exchanger means for furthercooling said precooled stream to at least its liquefaction temperature,at the superatmospheric pressure thereof, and totally liquefying saidprecooled feed stream by passing said feed stream in heat exchange withsaid first subcooled liquid fraction undergoing vaporization and then inheat exchange with said second subcooled liquid fraction undergoingvaporization,said methane-rich feed stream and said multicomponentrefrigerant being in indirect heat exchange with each other throughoutthe process. .Iaddend. .Iadd.
 27. A refrigeration system as claimed inclaim 26 further including conduit means connected to said fifth heatexchanger means and to said compressor for returning said vaporizedfirst and second fractions to said compressor as said multicomponentrefrigerant. .Iaddend..Iadd.
 28. A refrigeration system as claimed inclaim 27 further including conduit means connected to said fifth heatexchanger means for withdrawing said totally liquefied feed stream fromsaid fifth heat exchanger means. .Iaddend..Iadd.
 29. A refrigerationsystem as claimed in claim 28 including expansion means in said conduitmeans for expanding said totally liquefied feed stream. .Iaddend. .Iadd.30. A refrigeration system for liquefying the major portion of a gaseousmethane-rich feed stream at a superatmospheric pressure comprising incombination a. a first multi-stage heat exchanger means for precoolingsaid feed stream, b. means for supplying a multicomponent refrigerant,c. a compressor for compressing said multicomponent refrigerant to asuperatmospheric pressure, d. a second multi-stage heat exchanger meansfor cooling and to partially condense said multicomponent refrigerant,e. a phase separator connected to said second multistage heat exchangermeans for separating said partially condensed multicomponent refrigerantinto a single vapor fraction and a single condensed fraction, f. thirdheat exchanger means connected to said separator and including expansionmeans for subcooling said condensed liquid fraction in heat exchangewith itself, after expansion in said expansion means, to form a firstsubcooled liquid fraction, g. fourth heat exchanger means connected tosaid separator and including expansion means for liquefying andsubcooling said vapor fraction in heat exchange with said firstsubcooled liquid fraction, and with itself, after expansion in saidexpansion means, to form a second subcooled liquid fraction, and h.fifth heat exchanger means connected to said first heat exchanger meansfor further cooling said feed stream to at least -143° C. solely by heatexchange with said first and second subcooled liquid fractionsundergoing vaporization,said methane-rich feed stream and saidmulticomponent refrigerant being in indirect heat exchange with eachother throughout said system. .Iaddend. .Iadd.
 31. A refrigerationsystem as claimed in claim 30 including passage means connected to saidfifth heat exchanger means for returning said vaporized fractions tosaid compressor. .Iaddend..Iadd.
 32. A refrigeration system as claimedin claim 31 including expansion means connected to said fifth heatexchanger means for reducing the pressure of said cooled feed stream toa reduced pressure. .Iaddend. .Iadd.
 33. A refrigeration system forliquefying the major portion of a methane-rich feed stream comprisingthe combinationa. means supplying a multicomponent refrigerant includingone component having a boiling point substantially below that ofmethane, b. first heat exchanger means for precooling and partiallycondensing a substantial portion of said multicomponent refrigerant, c.a phase separator connected to said first heat exchanger means forseparating said partially condensed multicomponent refrigerant into avapor fraction and a condensed liquid fraction, d. second heat exchangermeans connected to said separator for subcooling said condensed liquidfraction in heat exchange with itself after expansion to form a firstsubcooled liquid fraction, e. third heat exchanger means connected tosaid separator for liquefying and subcooling said vapor fraction in heatexchange with said first subcooled liquid fraction, and with itselfafter expansion, to form a second subcooled liquid fraction, and f.fourth heat exchanger means for liquefying the major portion of saidprecooled feed stream in heat exchange with said first and secondsubcooled liquid fractions undergoing vaporization,said methane-richfeed stream and said multicomponent refrigerant being in indirect heatexchange with each other throughout the process. .Iaddend..Iadd.
 34. Ina refrigeration process wherein a multicomponent refrigerant is inindirect heat exchange throughout the process with a methane-rich feedstream and is partially condensed to form a single vapor fraction and asingle liquid fraction, the improvement which comprisesa. subcoolingsaid liquid fraction in heat exchange with itself after expansion toform a first subcooled liquid fraction, and b. liquefying and subcoolingall of said vapor fraction in heat exchange with said first subcooledliquid fraction, and with itself after expansion, to form a secondsubcooled liquid fraction. .Iaddend.